Methods for digesting nitro compounds have received interest as the products from the partial or total digestion of those nitro compounds have found an expanding variety of uses. These uses include drug intermediates, antibiotics, pesticides, herbicides, radiosensitizers and explosives which may be produced with nitros as starting materials. As used in these applications, the nitro compounds are partially or totally digested as part of the processing required for production of the final product.
Additionally, nitro compounds in many circumstances have been proven to create environmental or health hazards. For example, nitrobenzene has been shown to cause headaches, drowsiness, nausea, vomiting and methemoglobinemia with cyanosis. Nitrobenzene has also been shown to be toxic to rats with LD50 of 640 mg/kg.
Disposal of excess, aging, or unstable munitions (explosive nitro compounds) stockpiled around the world has been handled in various ways including openly burning/openly detonating (OB/OD) them, land burial, open sea disposal, and limited conversion or sale for use by the mining industry. However, most of these past practices are now unacceptable because of stringent environmental regulations. Specifically, OB/OD produces airborne particulates and pollutants and releases toxic compounds. Burial that is not sealed also releases toxic compounds into soil and groundwater. Additionally, the conversion of explosive nitro compounds from military explosives into mining-grade explosives is expensive and only removes a small percentage of a nation's stockpile from its inventory. Regulators allow incineration on a case-by-case basis with limits on how much can be burned. Thus, incineration is not a viable option for disposing of the currently large amount of explosive nitro compounds in the prevailing regulatory environment and is likewise viewed by the public as an unfriendly process.
The current stockpile of explosive nitro compounds requiring resource recovery or disposition (RRD) is 449,308 tons. Through 2001, over 1.2 million tons will pass through or reside in the RRD account (Joint Ordnance Commands Group; 1995). A totally different but significantly similar challenge exists in clean-up of the sites where soil and ground water are contaminated with TNT, RDX, HMX, and other explosive nitro compounds. Hence, there is a need for characterizing the reactions of explosive nitro compounds with naturally occurring enzymes, as well as a need for cost-effective technologies to degrade these explosive nitro compounds by simple and safe processes.
Such technologies include molten salt processes and supercritical water oxidation (SCWO). Molten salt is a high temperature process described in the U.S. Pat. No. 5,398,914 MOLTEN SALT PROCESS VESSEL. Supercritical water oxidation uses oxygen in water at a temperature and pressure above its critical point as described in U.S. Pat. No. 5,133,877 CONVERSION OF HAZARDOUS MATERIALS USING SUPERCRITICAL WATER OXIDATION. Disadvantages of large scale (demilitarization) and small scale (local explosive threat) deployment of both the molten salt and supercritical water oxidation include high energy cost, complexity of deployment, lack of mobility and reduced operational safety.
A process known as base hydrolysis has also been shown to render many explosive nitro compounds non-explosive via reaction with sodium hydroxide or ammonia. Base hydrolysis is another high temperature process described in the paper by Raymond L. Flesner et al., PILOT-SCALE BASE HYDROLYSIS PROCESSING OF HMX-BASED PLASTIC-BONDED EXPLOSIVES, .sub.4 th International Symposium On Special Topics In Chemical Propulsion: Challenges In Propellants And 100 Years After Nobel, May 27-31, 1996. Disadvantages of base hydrolysis include the need for expensive pressurized reactors to carry out the process at temperatures above the normal solution boiling point.
To take advantage of the potential uses of partially and totally digested explosive nitro compounds and to eliminate these compounds in circumstances where they pose environmental or health risks, a variety of processing schemes have been developed to bring about the partial or total digestion of these explosive nitro compounds. Many such schemes involve the use of naturally occurring enzymes to catalyze the digestion process. Such schemes are highly advantageous as the enzymes are often readily obtainable and their use as catalysts minimizes undesirable waste and byproducts. An example of the use of such enzymes is provided by Sommerville (Sommerville, C., Nishino, S.F., and Spain, J.C. (1995) J. Bacteriol., 177, 3837-3842), wherein it is demonstrated that digestion of nitrobenzene may result in phenylhydroxylamine as a byproduct through the use of oxygen insensitive nitrobenzene reductase as a catalyst. Schemes such as that described in Sommerville are characterized by an inability to control the digestion using a simple inhibitor such as molecular oxygen, as well as requirements to use an expensive digestive agent, NADPH. However, it is often desirable that the digestion of the explosive nitro compounds not be allowed to progress to completion such that partially digested products may be isolated and/or collected.
U.S. Pat. No. 5,777,190 to Shah et al., is directed to a method for the controlled digestion of explosive nitro compounds such as nitrobenzene and 2,4,6-trinitrotoluene (TNT) by enzymatic reaction with oxygen sensitive nitroreductase enzymes, such as ferredoxin NADP oxidoreductase. Through the addition of oxygen, the digestion of explosive nitro compounds may be halted at the point at which a partially digested product has been produced. Again, a disadvantage of this and prior processes is the relative unavailability and cost of expensive cofactors like nicotinamide adenine dinucleotide phosphate (NADPH) used for digesting the explosive nitro compounds.
Hence, there remains a need for a simple, safe, and cost-effective method to digest explosive nitro compounds using readily available digestive agents.